As a method of simultaneously achieving high efficiency and high power, which is normally a trade-off relationship in an inverter-controlled motor-generator, a method has been known wherein the voltage of a battery that supplies the power to the motor generator is made variable. According to this method, it is possible to reduce the loss and increase the efficiency of the inverter by using a low battery voltage when the load is low. On the other hand, if high output is required, high power can be generated by using a high battery voltage, thus making it possible to run current through the motor-generator while suppressing the counter electromotive force.
While a method has been devised to make the voltage variable using a DC-DC converter, it requires a reactor and also generates a switching loss so that it presents problems in terms of mountability and efficiency.
A method of making the battery voltage variable by means of switching the connection conditions (serial and parallel connections) of the battery is proposed in the Unexamined Japanese Patent Application Publication No. H5-236608.
However, as was the case in the aforementioned prior art, by simply switching the battery connection condition, abnormal current was generated due to an abrupt change of the voltage thus resulting in possible damage to the inverter, the motor generator, etc.